3'- and 4'-chloro-substituted analogs of benztropine: intravenous self-administration and in vitro radioligand binding studies in rhesus monkeys

Effects of benztropine analogs on delay discounting in rats

RATIONALE

Methylphenidate and d-amphetamine, medications used for treatment of attention deficit hyperactivity disorder (ADHD), are used recreationally and self-administered by laboratory animals. Benztropine (BZT) analogs, like those medications, increase synaptic dopamine levels but are less effective in maintaining self-administration, suggesting clinical utility with less abuse liability.

OBJECTIVES

The current study was designed to evaluate potential therapeutic effects of BZT analogs related to ADHD.

METHODS

Rats responded under a delay-discounting procedure in which responses on one lever produced immediate delivery of a single food pellet and alternative responses produced four food pellets either immediately or with various temporal delays, with those delays arranged in ascending or random orders in different groups of rats. Selection of the smaller more immediate reinforcer has been suggested as an aspect of "impulsivity," a trait with suggested involvement in ADHD. Other rats were studied under fixed-interval (FI) 300-s schedules to assess drug effects on behavior under temporal control.

RESULTS

d-Amphetamine, methylphenidate, and the BZT analog AHN 1-055, but not AHN 2-005 or JHW 007, increased selection of the large, delayed reinforcer with either arrangement of delays. All drugs changed the temporal distribution of responses within the FI from one with responses concentrated at the end to a more uniform distribution. Changes in the temporal distribution of FI responding occurred with drugs that did not affect discounting suggesting that discounting does not arise directly from the same temporal control processes controlling FI responding.

CONCLUSIONS

AHN 1-055 may be of clinical utility in the treatment of ADHD.

Model systems for analysis of dopamine transporter function and regulation

The dopamine transporter (DAT) plays a critical role in dopamine (DA) homeostasis by clearing transmitter from the extraneuronal space after vesicular release. DAT serves as a site of action for a variety of addictive and therapeutic reuptake inhibitors, and transport dysfunction is associated with transmitter imbalances in disorders such as schizophrenia, attention deficit hyperactive disorder, bipolar disorder, and Parkinson disease. In this review, we describe some of the model systems that have been used for in vitro analyses of DAT structure, function and regulation, and discuss a potential relationship between transporter kinetic values and membrane cholesterol.

The gold-standard in preclinical abuse liability testing: It's all relative

All new molecular entities (NMEs) with targeted or indirect effects on the central nervous system (CNS) must be evaluated for their abuse liability as a part of their nonclinical development plan. Inherently key in the drug control review is the term "relative abuse liability". The basis for determination of drug control is critically dependent on the nonclinical assessment of the reinforcing attributes of the NME in animals (rat is the regulatory preferred species) in a standard operant conditioning paradigm. Pharmaceutical representatives without a background in behavioral analysis or operant conditioning models must weigh through conceptually-intriguing language and constructs that accurately convey and communicate the relative potential for abuse to drug regulatory experts in the field. Effective statutory language in the preclinical assessment of relative abuse liabilities for schedule control status reviews must be 1) specific; 2) concise; 3) familiar to the regulators; 4) unambiguous; 5) constructive; and 6) formalized with respect to both international and national drug control policies. In this review we attempt to define and highlight the importance of the statutory language used to report self-administration study results to both parties engaged in NDA approval process.

Dopamine Transporter Dynamics of N-Substituted Benztropine Analogs with Atypical Behavioral Effects

Atypical dopamine transporter (DAT) inhibitors, despite high DAT affinity, do not produce the psychomotor stimulant and abuse profile of standard DAT inhibitors such as cocaine. Proposed contributing features for those differences include off-target actions, slow onsets of action, and ligand bias regarding DAT conformation. Several 3α-(4',4''-difluoro-diphenylmethoxy)tropanes were examined, including those with the following substitutions: N-(indole-3''-ethyl)- (GA1-69), N-(R)-2''-amino-3''-methyl-n-butyl- (GA2-50), N-2''aminoethyl- (GA2-99), and N-(cyclopropylmethyl)- (JHW013). These compounds were previously reported to have rapid onset of behavioral effects and were presently evaluated pharmacologically alone or in combination with cocaine. DAT conformational mode was assessed by substituted-cysteine accessibility and molecular dynamics (MD) simulations. As determined by substituted-cysteine alkylation, all BZT analogs except GA2-99 showed bias for a cytoplasmic-facing DAT conformation, whereas cocaine stabilized the extracellular-facing conformation. MD simulations suggested that several analog-DAT complexes formed stable R85-D476 "outer gate" bonds that close the DAT to extracellular space. GA2-99 diverged from this pattern, yet had effects similar to those of other atypical DAT inhibitors. Apparent DAT association rates of the BZT analogs in vivo were slower than that for cocaine. None of the compounds was self-administered or stimulated locomotion, and each blocked those effects of cocaine. The present findings provide more detail on ligand-induced DAT conformations and indicate that aspects of DAT conformation other than "open" versus "closed" may facilitate predictions of the actions of DAT inhibitors and may promote rational design of potential treatments for psychomotor-stimulant abuse.

σ Receptor Effects of N-Substituted Benztropine Analogs: Implications for Antagonism of Cocaine Self-Administration

Several N-substituted benztropine (BZT) analogs are atypical dopamine transport inhibitors as they have affinity for the dopamine transporter (DAT) but have minimal cocaine-like pharmacologic effects and can block numerous effects of cocaine, including its self-administration. Among these compounds, N-methyl (AHN1-055), N-allyl (AHN2-005), and N-butyl (JHW007) analogs of 3α-[bis(4'-fluorophenyl)methoxy]-tropane were more potent in antagonizing self-administration of cocaine and d-methamphetamine than in decreasing food-maintained responding. The antagonism of cocaine self-administration (0.03-1.0 mg/kg per injection) with the above BZT analogs was reproduced in the present study. Further, the stimulant-antagonist effects resembled previously reported effects of pretreatments with combinations of standard DAT inhibitors and σ1-receptor (σ1R) antagonists. Therefore, the present study examined binding of the BZT analogs to σRs, as well as their in vivo σR antagonist effects. Each of the BZT analogs displaced radiolabeled σR ligands with nanomolar affinity. Further, self-administration of the σR agonist DTG (0.1-3.2 mg/kg/injection) was dose dependently blocked by AHN2-005 and JHW007 but potentiated by AHN1-055. In contrast, none of the BZT analogs that were active against DTG self-administration was active against the self-administration of agonists at dopamine D1-like [R(+)-SKF 81297, (±)-SKF 82958 (0.00032-0.01 mg/kg per injection each)], D2-like [R(-)-NPA (0.0001-0.0032 mg/kg per injection), (-)-quinpirole (0.0032-0.1 mg/kg per injection)], or μ-opioid (remifentanil, 0.0001-0.0032 mg/kg per injection) receptors. The present results indicate that behavioral antagonist effects of the N-substituted BZT analogs are specific for abused drugs acting at the DAT and further suggest that σR antagonism contributes to those actions.

2-Substituted 3β-Aryltropane Cocaine Analogs Produce Atypical Effects without Inducing Inward-Facing Dopamine Transporter Conformations

Previous structure-activity relationship studies indicate that a series of cocaine analogs, 3β-aryltropanes with 2β-diarylmethoxy substituents, selectively bind to the dopamine transporter (DAT) with nanomolar affinities that are 10-fold greater than the affinities of their corresponding 2α-enantiomers. The present study compared these compounds to cocaine with respect to locomotor effects in mice, and assessed their ability to substitute for cocaine (10 mg/kg, i.p.) in rats trained to discriminate cocaine from saline. Despite nanomolar DAT affinity, only the 2β-Ph2COCH2-3β-4-Cl-Ph analog fully substituted for cocaine-like discriminative effects. Whereas all of the 2β compounds increased locomotion, only the 2β-(4-ClPh)PhCOCH2-3β-4-Cl-Ph analog had cocaine-like efficacy. None of the 2α-substituted compounds produced either of these cocaine-like effects. To explore the molecular mechanisms of these drugs, their effects on DAT conformation were probed using a cysteine-accessibility assay. Previous reports indicate that cocaine binds with substantially higher affinity to the DAT in its outward (extracellular)- compared with inward-facing conformation, whereas atypical DAT inhibitors, such as benztropine, have greater similarity in affinity to these conformations, and this is postulated to explain their divergent behavioral effects. All of the 2β- and 2α-substituted compounds tested altered cysteine accessibility of DAT in a manner similar to cocaine. Furthermore, molecular dynamics of in silico inhibitor-DAT complexes suggested that the 2-substituted compounds reach equilibrium in the binding pocket in a cocaine-like fashion. These behavioral, biochemical, and computational results show that aryltropane analogs can bind to the DAT and stabilize outward-facing DAT conformations like cocaine, yet produce effects that differ from those of cocaine.

Behavioral, biological, and chemical perspectives on atypical agents targeting the dopamine transporter

BACKGROUND

Treatment of stimulant-use disorders remains a formidable challenge, and the dopamine transporter (DAT) remains a potential target for antagonist or agonist-like substitution therapies.

METHODS

This review focuses on DAT ligands, such as benztropine, GBR 12909, modafinil, and DAT substrates derived from phenethylamine or cathinone that have atypical DAT-inhibitor effects, either in vitro or in vivo. The compounds are described from a molecular mechanistic, behavioral, and medicinal-chemical perspective.

RESULTS

Possible mechanisms for atypicality at the molecular level can be deduced from the conformational cycle for substrate translocation. For each conformation, a crystal structure of a bacterial homolog is available, with a possible role of cholesterol, which is also present in the crystal of Drosophila DAT. Although there is a direct relationship between behavioral potencies of most DAT inhibitors and their DAT affinities, a number of compounds bind to the DAT and inhibit dopamine uptake but do not share cocaine-like effects. Such atypical behavior, depending on the compound, may be related to slow DAT association, combined sigma-receptor actions, or bias for cytosol-facing DAT. Some structures are sterically small enough to serve as DAT substrates but large enough to also inhibit transport. Such compounds may display partial DA releasing effects, and may be combined with release or uptake inhibition at other monoamine transporters.

CONCLUSIONS

Mechanisms of atypical DAT inhibitors may serve as targets for the development of treatments for stimulant abuse. These mechanisms are novel and their further exploration may produce compounds with unique therapeutic potential as treatments for stimulant abuse.

The stereotypy-inducing effects of N-substituted benztropine analogs alone and in combination with cocaine do not account for their blockade of cocaine self-administration

RATIONALE

Previous studies have demonstrated that several N-substituted 4', 4″-diF-benztropine (BZT) analogs with high dopamine transporter affinity selectively decreased cocaine self-administration without affecting food-maintained behavior in rats.

OBJECTIVES

The present study examined if the decreases in cocaine self-administration are due to competition from excess behavioral activity (hyperlocomotion or stereotypy) induced by the BZT analogs alone or in combination with cocaine.

RESULTS

Pretreatments with the typical dopamine uptake inhibitor methylphenidate [1.0, 3.2, and 10 mg/kg, intraperitoneally (i.p.)] dose-dependently shifted the cocaine self-administration dose-effect curve (0, 0.032, 0.1, 0.32, and 1.0 mg/kg/injection) leftward. The shift in the dose-effect curve was obtained at doses of methylphenidate that, when administered alone, also decreased food-maintained behavior and increased locomotor activity and stereotypy. In contrast, the N-substituted BZT analogs, JHW 007 (1.0, 3.2, and 10 mg/kg, i.p.), AHN 1-055 (10 mg/kg), and, AHN 2-005 (10 mg/kg), as previously reported, decreased the maximum for the cocaine self-administration dose-effect curve, and did so at doses that were virtually without effects on food-maintained behavior. Further, the BZT analogs alone had minimal effects on locomotor activity and stereotypies and did not appreciably change the effects of cocaine on these measures when administered in combination with cocaine.

CONCLUSIONS

The present results suggest that the decrease in cocaine self-administration produced by the N-substituted BZT analogs is due to an antagonism of the reinforcing effects of cocaine rather than due to interference from competing behavioral overstimulation, and further supports the development of N-substituted BZT analogs as medications to treat cocaine abuse.

A selective dopamine reuptake inhibitor improves prefrontal cortex-dependent cognitive function: potential relevance to attention deficit hyperactivity disorder

Drugs used to treat attention deficit hyperactivity disorder (ADHD) improve prefrontal cortex (PFC)-dependent cognitive function. The majority of ADHD-related treatments act either as dual norepinephrine (NE) and dopamine (DA) reuptake inhibitors (psychostimulants) or selective NE reuptake inhibitors (SNRIs). Certain benztropine analogs act as highly selective DA reuptake inhibitors while lacking the reinforcing actions, and thus abuse potential, of psychostimulants. To assess the potential use of these compounds in the treatment of ADHD, we examined the effects of a well-characterized benztropine analog, AHN 2-005, on performance of rats in a PFC-dependent delayed-alternation task of spatial working memory. Similar to that seen with all drugs currently approved for ADHD, AHN 2-005 dose-dependently improved performance in this task. Clinically-relevant doses of psychostimulants and SNRIs elevate NE and DA preferentially in the PFC. Despite the selectivity of this compound for the DA transporter, additional microdialysis studies demonstrated that a cognition-enhancing dose of AHN 2-005 that lacked locomotor activating effects increased extracellular levels of both DA and NE in the PFC. AHN 2-005 produced a larger increase in extracellular DA in the nucleus accumbens, although the magnitude of this was well below that seen with motor activating doses of psychostimulants. Collectively, these observations suggest that benztropine analogs may be efficacious in the treatment of ADHD or other disorders associated with PFC dysfunction. These studies provide a strong rationale for future research focused on the neural mechanisms contributing to the cognition-enhancing actions and the potential clinical utility of AHN 2-005 and related compounds. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

N-substituted benztropine analogs: selective dopamine transporter ligands with a fast onset of action and minimal cocaine-like behavioral effects

Previous studies suggested that differences between the behavioral effects of cocaine and analogs of benztropine were related to the relatively slow onset of action of the latter compounds. Several N-substituted benztropine analogs with a relatively fast onset of effects were studied to assess whether a fast onset of effects would render the effects more similar to those of cocaine. Only one of the compounds increased locomotor activity, and the increases were modest compared with those of 10 to 20 mg/kg cocaine. In rats trained to discriminate 10 mg/kg cocaine from saline none of the compounds produced more than 40% cocaine-like responds up to 2 h after injection. None of the compounds produced place-conditioning when examined up to 90 min after injection, indicating minimal abuse liability. The compounds had 5.6 to 30 nM affinities at the dopamine transporter (DAT), with uniformly lower affinities at norepinephrine and serotonin transporters (from 490-4600 and 1420-7350 nM, respectively). Affinities at muscarinic M(1) receptors were from 100- to 300-fold lower than DAT affinities, suggesting minimal contribution of those sites to the behavioral effects of the compounds. Affinities at histaminic H(1) sites were from 11- to 43-fold lower than those for the DAT. The compounds also had affinity for sigma, 5-hydroxytryptamine(1) (5-HT(1)), and 5-HT(2) receptors that may have contributed to their behavioral effects. Together, the results indicate that a slow onset of action is not a necessary condition for reduced cocaine-like effects of atypical DAT ligands and suggest several mechanisms that may contribute to the reduced cocaine-like efficacy of these compounds.

Dopamine transporter-dependent and -independent striatal binding of the benztropine analog JHW 007, a cocaine antagonist with low abuse liability

The benztropine analog N-(n-butyl)-3α-[bis(4'-fluorophenyl)methoxy]-tropane (JHW 007) displays high affinity for the dopamine transporter (DAT), but unlike typical DAT ligands, has relatively low abuse liability and blocks the effects of cocaine, including its self-administration. To determine sites responsible for the cocaine antagonist effects of JHW 007, its in vitro binding was compared with that of methyl (1R,2S,3S,5S)-3-(4-fluorophenyl)-8-methyl-8-azabicyclo[3.2.1]octane-2-carboxylate (WIN 35428) in rats, mice, and human DAT (hDAT)-transfected cells. A one-site model, with K(d) values of 4.21 (rat) and 8.99 nM (mouse) best fit the [(3)H]WIN 35428 data. [(3)H]JHW 007 binding best fit a two-site model (rat, 7.40/4400 nM; mouse, 8.18/2750 nM), although a one-site fit was observed with hDAT membranes (43.7 nM). Drugs selective for the norepinephrine and serotonin transporters had relatively low affinity in competition with [(3)H]JHW 007 binding, as did drugs selective for other sites identified previously as potential JHW 007 binding sites. The association of [(3)H]WIN 35428 best fit a one-phase model, whereas the association of [(3)H]JHW 007 best fit a two-phase model in all tissues. Because cocaine antagonist effects of JHW 007 have been observed previously soon after injection, its rapid association observed here may contribute to those effects. Multiple [(3)H]JHW 007 binding sites were obtained in tissue from mice lacking the DAT, suggesting these as yet unidentified sites as potential contributors to the cocaine antagonist effects of JHW 007. Unlike WIN 35428, the binding of JHW 007 was Na(+)-independent. This feature of JHW 007 has been linked to the conformational status of the DAT, which in turn may contribute to the antagonism of cocaine.

Discovery of drugs to treat cocaine dependence: behavioral and neurochemical effects of atypical dopamine transport inhibitors

Stimulant drugs acting at the dopamine transporter (DAT), like cocaine, are widely abused, yet effective medical treatments for this abuse have not been found. Analogs of benztropine (BZT) that, like cocaine, act at the DAT have effects that differ from cocaine and in some situations block the behavioral, neurochemical, and reinforcing actions of cocaine. Neurochemical studies of dopamine levels in brain and behavioral studies have demonstrated that BZT analogs have a relatively slow onset and reduced maximal effects compared to cocaine. Pharmacokinetic studies, however, indicated that the BZT analogs rapidly access the brain at concentrations above their in vitro binding affinities, while binding in vivo demonstrates apparent association rates for BZT analogs lower than that for cocaine. Additionally, the off-target effects of these compounds do not fully explain their differences from cocaine. Initial structure-activity studies indicated that BZT analogs bind to DAT differently from cocaine and these differences have been supported by site-directed mutagenesis studies of the DAT. In addition, BZT analog-mediated inhibition of uptake was more resistant to mutations producing inward conformational DAT changes than cocaine analogs. The BZT analogs have provided new insights into the relation between the molecular and behavioral actions of cocaine and the diversity of effects produced by dopamine transport inhibitors. Novel interactions of BZT analogs with the DAT suggest that these drugs may have a pharmacology that would be useful in their development as treatments for cocaine abuse.

Assessment of reinforcing effects of benztropine analogs and their effects on cocaine self-administration in rats: comparisons with monoamine uptake inhibitors

Benztropine (BZT) analogs inhibit dopamine uptake but are less effective than cocaine in producing behavioral effects predicting abuse liability. The present study compared reinforcing effects of intravenous BZT analogs with those of standard monoamine uptake inhibitors and the effects of their oral pretreatment on cocaine self-administration. Responding of rats was maintained by cocaine [0.032-1.0 mg/kg/injection (inj)] or food reinforcement under fixed-ratio five-response schedules. Maximal rates of responding were maintained by 0.32 mg/kg/inj cocaine or substituted methylphenidate, with lower rates maintained at lower and higher doses. The N-methyl BZT analog, AHN 1-055 (3alpha-[bis(4'-fluorophenyl)methoxy]-tropane), also maintained responding (0.1 mg/kg/inj), although maximal rates were less than those with cocaine. Responding was not maintained above vehicle levels by the N-allyl, AHN 2-005 (N-allyl-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane), and N-butyl, JHW 007 [N-(n-butyl)-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane], BZT analogs, and it was not maintained with nisoxetine or citalopram. Presession treatment with methylphenidate (3.2-32 mg/kg) dose-dependently shifted the cocaine self-administration dose-effect curve leftward, whereas nisoxetine and citalopram effects were not significant. An intermediate dose of AHN 1-055 (32 mg/kg) increased responding maintained by low cocaine doses and decreased responding maintained by higher doses. A higher dose of AHN 1-055 completely suppressed cocaine-maintained responding. Both AHN 2-005 and JHW 007 dose-dependently (10-32 mg/kg) decreased cocaine self-administration, shifting its dose-effect curve down. Decreases in cocaine-maintained responding occurred at doses of methylphenidate and BZT analogs that left food-maintained responding unchanged. During a component in which injections were not available, methylphenidate and AHN 1-055, but not AHN 2-005 or JHW 007, increased response rates. These findings further support the low abuse liability of BZT analogs and their potential development as medications for cocaine abuse.

Population pharmacokinetics, brain distribution, and pharmacodynamics of 2nd generation dopamine transporter selective benztropine analogs developed as potential substitute therapeutics for treatment of cocaine abuse

A second generation of N-substituted 3alpha-[bis(4'-fluorophenyl)methoxy]-tropanes (GA 1-69, JHW 005 and JHW 013) binds with high affinity to the dopamine transporter (DAT) and are highly selective toward DAT compared to muscarinic receptor binding (M1). The objective of this study was to characterize brain distribution, pharmacokinetics, and pharmacodynamics [extracellular brain dopamine (DA) levels] of three novel N-substituted benztropine (BZT) analogs in male Sprague-Dawley rats. The BZT analogs displayed a higher distribution (Vd = 8.69-34.3 vs. 0.9 L/kg) along with longer elimination (t l/2: 4.1-5.4 vs. 0.5 h) than previously reported for cocaine. Brain-to-plasma partition coefficients were 1.3-2.5 vs. 2.1 for cocaine. The effect of the BZT analogs on extracellular brain (DA) levels ranged from minimal effects (GA 1-69) to several fold elevation (approximately 850% of basal DA for JHW 013) at the highest dose evaluated. PK/PD analysis of exposure-response data resulted in lower IC50 values for the BZT analogs compared to cocaine indicating their higher potency to inhibit DA reuptake (0.1-0.3 vs. 0.7 mg/L). These BZT analogs possess significantly different PK and PD profiles as compared to cocaine suggesting that further evaluation as cocaine abuse therapeutics is warranted.

Dopamine transport inhibitors based on GBR12909 and benztropine as potential medications to treat cocaine addiction

The discovery and development of medications to treat addiction and notably, cocaine addiction, have been frustrated by both the complexity of the disorder and the lack of target validation in human subjects. The dopamine transporter has historically been a primary target for cocaine abuse medication development, but addictive liability and other confounds of such inhibitors of dopamine uptake have limited clinical evaluation and validation. Herein we describe efforts to develop analogues of the dopamine uptake inhibitors GBR 12909 and benztropine that show promising profiles in animal models of cocaine abuse that contrast to that of cocaine. Their unique pharmacological profiles have provided important insights into the reinforcing actions of cocaine and we propose that clinical investigation of novel dopamine uptake inhibitors will facilitate the discovery of cocaine-abuse medications.

Muscarinic preferential M(1) receptor antagonists enhance the discriminative-stimulus effects of cocaine in rats

Previous studies of benztropine analogues have found them to inhibit dopamine uptake like cocaine, but with less effectiveness than cocaine in producing behavioral effects related to drug abuse. Studies have assessed whether nonselective muscarinic antagonists decrease the effects of cocaine because many of the benztropine analogues are also muscarinic antagonists. As previous studies were conducted with nonselective muscarinic antagonists and the benztropine analogues show preferential affinity for the M(1) muscarinic receptor subtype, the present study examined interactions of cocaine and the preferential M(1) antagonists, telenzepine (TZP) and trihexyphenidyl (TXP) on subjective effects in rats trained to discriminate cocaine (10 mg/kg, i.p.) from saline injections. Cocaine dose-dependently increased the percentage of responses on the cocaine-appropriate lever, with full substitution at the training dose. In contrast neither TZP nor TXP produced more than 25% cocaine-appropriate responding at any dose. Both M(1) antagonists produced significant leftward shifts in the cocaine dose-effect curve, TZP at 3.0 and TXP at 0.3 and 1.0 mg/kg. The present results indicate that preferential antagonist actions at muscarinic M(1) receptors enhance rather than attenuate the discriminative-stimulus effects of cocaine, and thus those actions unlikely contribute to the reduced cocaine-like effects of BZT analogues.

Applicability of the dopamine and rate hypotheses in explaining the differences in behavioral pharmacology of the chloro-benztropine analogs: studies conducted using intracerebral microdialysis and population pharmacodynamic modeling

Previous studies indicated that the chloro-benztropine analogs differed significantly in their cocaine-like activity, which was not expected based on the similarity in their in vitro binding affinity and functional potency at the dopamine transporter (DAT). The present study was designed to extend the understanding of the involvement of both pharmacokinetic and pharmacodynamic factors in mediating the behavioral differences among these analogs. The pharmacokinetics of 3'-chloro-3alpha-(diphenylmethoxy)tropane (3'-Cl BZT), the analog showing a cocaine-like behavioral profile in rodents, was compared with previously reported pharmacokinetic characteristics of cocaine and 4',4''-dichloro-3alpha-(diphenylmethoxy)tropane (4',4''-diCl BZT), an analog totally devoid of cocaine-like actions. Microdialysis studies in rats were conducted to determine whether 3'-Cl and 4',4''-diCl BZT differed significantly in their effect on nucleus accumbens extracellular dopamine levels, with cocaine serving as a reference. A mechanistic model based on DAT association/dissociation kinetics was used to describe the time delay between the plasma concentrations of the chloro-analogs and their dopaminergic effects. 3'-Cl BZT had plasma elimination half-life of 1.9 h versus 0.5 and 21.1 h for cocaine and 4',4''-diCl BZT, respectively. 4',4''-diCl BZT increased the DA levels at a slower rate and to a significantly lower extent relative to 3'-Cl BZT that were, in turn, lower than cocaine. The duration of dopamine elevation was as follows: 4',4''-diCl BZT > 3'-Cl BZT > cocaine. The model indicated faster association and dissociation with DAT for 3'-Cl BZT relative to 4',4''-diCl BZT. The present results indicate that behavioral differences among the chloro-analogs may be explainable based on both the dopamine and rate hypotheses of drug abuse.

Effects of Muscarinic M1 Receptor Blockade on Cocaine-Induced Elevations of Brain Dopamine Levels and Locomotor Behavior in Rats

Cholinergic muscarinic systems have been shown to influence dopaminergic function in the central nervous system. In addition, previous studies of benztropine analogs that inhibit dopamine uptake and show antagonism at muscarinic receptors show these drugs to be less effective than cocaine in producing its various prototypic effects such as locomotor stimulation. Because previous pharmacological studies on these topics have used nonselective M1 antagonists, we examined the interactions of preferential M1 muscarinic antagonists and cocaine. Dose-dependent increases in extracellular levels of dopamine in selected brain areas, the nucleus accumbens (NAc) shell and core, and the prefrontal cortex, were produced by cocaine but not by the preferential M1 antagonists telenzepine and trihexyphenidyl. When administered with cocaine, however, both M1 antagonists dose-dependently increased the effects of cocaine on dopamine in the NAc shell, and these effects were selective in that they were not obtained in the NAc core or in the prefrontal cortex. Telenzepine also increased locomotor activity, although the effect was small compared with that of cocaine. The locomotor stimulant effects of trihexyphenidyl, in contrast, approached those of cocaine. Telenzepine attenuated, whereas trihexyphenidyl enhanced the locomotor stimulant effects of cocaine, with neither drug facilitating cocaine-induced stereotypy. The present results indicate that preferential antagonist effects at muscarinic M1 receptors do not uniformly alter all of the effects of cocaine, nor do they explain the differences in effects of cocaine and benztropine analogs, and that the alterations in dopamine levels in the NAc shell do not predict the behavioral effects of the interactions with cocaine.

Benztropine pretreatment does not affect responses to acute cocaine administration in human volunteers

Benztropine (Cogentin ) was evaluated for its ability to block cocaine's physiological and subjective effects in humans. In healthy, recreational users of cocaine, placebo, or benztropine (1, 2, and 4 mg orally) was given 2 hr before subjects self-administered 0.9 mg/kg of cocaine intranasally. Measurements were made for 2 hr following cocaine administration, and plasma cocaine and cocaine metabolites were assayed. Cocaine produced typical increases in heart rate and alterations in self-reports measured by visual analog scales (VAS). Benztropine alone did not produce changes on any of these measures. Responses to cocaine with and without benztropine pretreatment were similar: benztropine did not change cocaine's effects. This study of one of the tropane-ring analogs that is approved for human use suggests this compound does not alter cocaine-induced effects, but just as importantly, does not produce any adverse behavioral or physiological effects. The exact therapeutic application of benztropine as a possible adjunct treatment for cocaine abuse in humans require further exploration.

Transport, Metabolism, and in Vivo Population Pharmacokinetics of the Chloro Benztropine Analogs, a Class of Compounds Extensively Evaluated in Animal Models of Drug Abuse

Recently, extensive behavioral research has been conducted on the benztropine (BZT) analogs with the goal of developing successful therapeutics for cocaine abuse. The present study was conducted to characterize the contribution of dispositional factors in mediating the behavioral differences among the chloro BZT analogs and to identify cytochrome P450 enzymes involved in their metabolism. Bidirectional transport and efflux studies of four of the chloro BZT analogs were conducted. Screening with a panel of human and rat Supersomes was performed for 4',4''-diCl BZT. In addition, pharmacokinetic and brain distribution studies for 4'-Cl and 4',4''-diCl BZT in Sprague-Dawley rats were conducted. The permeability of the chloro analogs ranged from 8.26 to 32.23 and from 1.37 to 21.65 x 10(-6) cm/s, whereas the efflux ratios ranged from 2.1 to 6.9 and from 3.3 to 28.4 across Madin-Darby canine kidney-multidrug resistance 1 (MDCK-MDR1) and Caco-2 monolayers, respectively. The P-glycoprotein (P-gp) inhibitor verapamil reduced the efflux ratios and enhanced the absorptive transport of the chloro BZT analogs. 4',4''-diCl BZT was a substrate of human CYP2D6 and 2C19 and rat 2C11 and 3A1. The brain uptake for 4'-Cl and 4',4''-diCl BZT was comparable and higher than previously reported for cocaine (brain-to-plasma partition coefficient = 4.6-4.7 versus 2.1 for cocaine). The rank order for t(1/2) was 4',4''-diCl BZT >> 4'-Cl BZT > cocaine and for steady-state volume of distribution was 4'-Cl BZT > 4',4''-diCl BZT >> cocaine. In conclusion, the chloro analogs differ significantly in their clearance and duration of action, which correlates to their behavioral profiles and abuse liability. Furthermore, these results suggest that the distinctive behavioral profile of these analogs is not due to limited brain exposure.

Gain of function mutants reveal sites important for the interaction of the atypical inhibitors benztropine and bupropion with monoamine transporters

Two atypical inhibitors of the dopamine transporter, benztropine, used in the treatment of Parkinson's disease, and bupropion, used as an antidepressant, show very different psychostimulant effects when compared with another inhibitor, cocaine. Taking advantage of the differential sensitivity of the dopamine and the norepinephrine transporters (DAT and NET) to benztropine and bupropion, we have used site-directed mutagenesis to produce gain-of-function mutants in NET which demonstrate that Ala279 in the trans-membrane domain 5 (TM5) and Ser359 in the TM7 of DAT are responsible for the higher sensitivity of DAT to both bupropion and benztropine. Substitution of these two DAT residues into the NET background does not alter the potency of NET-selective inhibitors, such as desipramine. The results from experiments examining the ability of DAT-selective inhibitors to displace [3H]nisoxetine binding in NET gain-of-function mutants suggest that Ser359 contributes to the initial binding of the inhibitor, and that Ala279 may influence subsequent steps involved in the blockade of translocation. Thus, these studies begin to identify residues that are important for the unique molecular interactions of benztropine and bupropion with the DAT, and that ultimately may contribute to the distinct behavioral actions of these drugs.

Place Conditioning and Locomotor Effects of N-Substituted, 4′,4′′-Difluorobenztropine Analogs in Rats

Previous studies demonstrated that analogs of benztropine [3alpha-(diphenyl-methoxy)tropane (BZT)] bind to the dopamine (DA) transporter with high affinity, inhibit DA uptake, but do not maintain rates of responding in self-administration procedures comparable with those maintained by cocaine. Some BZT analogs have an onset of action that is slower than that for cocaine that may contribute to this decreased effectiveness. In addition, some BZT analogs have affinity for muscarinic-M1 receptors that may interfere with reinforcing effects. The present study assessed effects of BZT analogs in place-conditioning procedures designed to accommodate variations in onset of effect. BZT analogs with variations in relative affinities for the DA transporter over M1 receptors from equal [AHN 1-055 (3alpha-[bis(4'-fluorophenyl)methoxy]-tropane)] to 16-fold [JHW 007 (N-(n-butyl)-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane)] were compared with cocaine and the muscarinic antagonist, atropine. Cocaine (10-20 mg/kg) but not atropine (1.0-5.6 mg/kg) produced dose-related place conditioning. The N-methyl-substituted BZT analog, AHN 1-055, was without significant effects at doses that ranged from 0.3 to 3.0 mg/kg and when administered up to 90 min before conditioning trials. In contrast, effects of AHN 2-005 (N-allyl-3alpha-[bis(4'-fluorophenyl)methoxy]-tropane; 0.1-10.0 mg/kg) were significant, and those of JHW 007 approached significance when administered 45 min but not immediately or 90 min before trials. Atropine blocked the effect of AHN 2-005 and approached significant antagonism of cocaine. The present study further supports and extends previous results showing minimal preclinical indications of abuse liability of BZT analogs and suggests that these differences from cocaine are not entirely accounted for by a slower onset of action or muscarinic M1 receptor affinity.

Effects of 4′-Chloro-3α-(diphenylmethoxy)-tropane on Mesostriatal, Mesocortical, and Mesolimbic Dopamine Transmission: Comparison with Effects of Cocaine

Increase in dopamine (DA) neurotransmission resulting from blockade of the DA transporter (DAT) after administration of cocaine is believed to play a major role in mediating its behavioral and reinforcing effects. Since it was hypothesized that drugs that block the DAT have cocaine-like behavioral effects, it was of interest to study in the present article the stimulant effects of cocaine on locomotor activity and on pattern of activation of DA neurotransmission in different DAergic terminal areas in rats and compare these effects with those of 4'-chloro-3alpha-(diphenylmethoxy)-tropane (4-Cl-BZT), a benztropine analog showing higher affinity for the DAT, but reduced behavioral effects compared with cocaine. Administration of cocaine resulted in a dose-dependent stimulation of locomotor activity and DA neurotransmission in the nucleus accumbens shell and core, dorsal caudate, and in the medial prefrontal cortex (PFCX) measured by microdialysis. At comparable doses, the effects of 4-Cl-BZT on DA levels in all brain areas except the PFCX were generally reduced compared with those of cocaine, as were the effects on locomotor activity. The differences in behavioral effects corresponded generally to differences between the drugs with regard to their stimulation of extracellular DA levels, although the mechanism(s) for the differences in extracellular DA may involve effects mediated by sites other than the DAT or differences in the efficiency of the two drugs in blocking DA uptake. Nonetheless, the present results suggest that the differences in behavioral effects between cocaine and 4-Cl-BZT are related to differences in their patterns of activation of DA transmission.

Combinatorial synthesis of benztropine libraries and their evaluation as monoamine transporter inhibitors

A combinatorial synthesis of benztropine analogues is presented. Radical azidonation of 3-benzyloxy-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 3 to 3-(1-azidobenzyloxy)-8-azabicyclo[3.2.1]octane-8-carboxylic acid tert-butyl ester 4 was used as a key step in the synthesis. This step was optimized by adding 10% DMF to the reaction. Reaction of 4 with phenyl magnesium bromide followed by Boc removal and N-methylation gave benztropine 1. Reaction of five-component Grignard reagents with 4 was used to create a two-dimensional library of 25 N-normethylbenztropine analogues. Further reaction of this library with five alkyl bromides was carried out to create a three-dimensional library containing 125 compounds. Screening of the libraries towards binding and inhibition of uptake of the human dopamine (hDAT), serotonin (hSERT) and norepinephrine transporters (hNET) was carried out. None of the synthesized compounds were found to be stronger than benztropine, and none were selective for inhibition of binding over monoamine uptake.

Effects of N-substituted analogs of benztropine: diminished cocaine-like effects in dopamine transporter ligands

Previous studies demonstrated that analogs of benztropine (BZT) possess high affinity for the dopamine transporter, inhibit dopamine uptake, but generally have behavioral effects different from those of cocaine. One hypothesis is that muscarinic-M(1) receptor actions interfere with cocaine-like effects. Several tropane-nitrogen substitutions of 4',4"-diF-BZT have reduced M(1) affinity compared with the CH(3)-analog (AHN 1-055; 3alpha-[bis-(4-fluorophenyl)methoxy]tropane). All of the compounds displaced [(3)H]WIN 35,428 (2beta-carbomethoxy-3beta-(4-fluorophenyl)tropane) binding with affinities ranging from 11 to 108 nM. Affinities at norepinephrine ([(3)H]nisoxetine) and serotonin ([(3)H]citalopram) transporters ranged from 457 to 4810 and 376 to 3260 nM, respectively, and at muscarinic M(1) receptors ([(3)H]pirenzepine) from 11.6 (AHN 1-055) to higher values, reaching 1030 nM for the other BZT-analogs. Cocaine and AHN 1-055 produced dose-related increases in locomotor activity in mice, with AHN 1-055 less effective than cocaine. The other compounds were ineffective in stimulating activity. In rats discriminating cocaine (29 micromol/kg i.p.) from saline, WIN 35,428 fully substituted for cocaine, whereas AHN 1-055 produced a maximal substitution of 79%. None of the other analogs fully substituted for cocaine. WIN 35,428 produced dose-related leftward shifts in the cocaine dose-effect curve, whereas selected BZT analogs produced minimal changes in the effects of cocaine. The results suggest that reducing M(1) affinity of 4',4"-diF-BZT with N-substitutions reduces effectiveness in potentiating the effects of cocaine. Furthermore, although the BZT-analogs bind with high affinity at the dopamine transporter, their behavioral effects differ from those of cocaine. These compounds have reduced efficacy compared with cocaine, a long duration of action, and may serve as leads for the development of medications to treat cocaine abuse.

Agents in Development for the Management of Cocaine Abuse

Cocaine abuse is a serious health problem in many areas of the world, yet there are no proven effective medications for the treatment of cocaine dependence. Preclinical studies suggest that the reinforcing effect of cocaine that promotes its abuse is mediated by blockade of the presynaptic dopamine transporter. This results in increased dopamine activity in the mesolimbic or meso-accumbens dopamine reward system of brain. Development of new medications to treat cocaine dependence has focused on manipulation of this dopamine system, either by direct action on dopamine binding sites (transporter or receptors) or indirectly by affecting other neurotransmitter systems that modulate the dopamine system. In principle, a medication could act via one of three mechanisms: (i) as a substitute for cocaine by producing similar dopamine effects; (ii) as a cocaine antagonist by blocking the binding of cocaine to the dopamine transporter; or (iii) as a modulator of cocaine effects by acting at other than the cocaine binding site. The US National Institute on Drug Abuse has a Clinical Research Efficacy Screening Trial (CREST) programme to rapidly screen existing medications. CREST identified four medications warranting phase II controlled clinical trials: cabergoline, reserpine, sertraline and tiagabine. In addition, disulfiram and selegiline (deprenyl) have been effective and well tolerated in phase II trials. However, selegiline was found ineffective in a recent phase III trial. Promising existing medications probably act via the first or third aforementioned mechanisms. Sustained-release formulations of stimulants such as methylphenidate and amfetamine (amphetamine) have shown promise in a stimulant substitution approach. Disulfiram and selegiline increase brain dopamine concentrations by inhibition of dopamine-catabolising enzymes (dopamine-beta-hydroxylase and monoamine oxidase B, respectively). Cabergoline is a direct dopamine receptor agonist, while reserpine depletes presynaptic stores of dopamine (as well as norepinephrine and serotonin). Sertraline, baclofen and vigabatrin indirectly reduce dopamine activity by increasing activity of neurotransmitters (serotonin and GABA) that inhibit dopamine activity. Promising new medications act via the second or third aforementioned mechanisms. Vanoxerine is a long-acting inhibitor of the dopamine transporter which blocks cocaine binding and reduces cocaine self-administration in animals. Two dopamine receptor ligands that reduce cocaine self-administration in animals are also undergoing phase I human safety trials. Adrogolide is a selective dopamine D(1) receptor agonist; BP 897 is a D(3) receptor partial agonist.A pharmacokinetic approach to treatment would block the entry of cocaine into the brain or enhance its catabolism so that less cocaine reached its site of action. This is being explored in animals using the natural cocaine-metabolising enzyme butyrylcholinesterase (or recombinant versions with enhanced capabilities), catalytic antibodies, and passive or active immunisation to produce anti-cocaine binding antibodies. A recent phase I trial of a "cocaine vaccine" found it to be well tolerated and producing detectable levels of anti-cocaine antibodies for up to 9 months after immunisation.

Evaluation of the blood-brain barrier transport, population pharmacokinetics, and brain distribution of benztropine analogs and cocaine using in vitro and in vivo techniques

The N-substituted 3alpha-[bis(4'-fluorophenyl)methoxy]tropanes (AHN 2-003, AHN 1-055, AHN 2-005, and JHW 007) bind with high affinity to the dopamine transporter and inhibit dopamine uptake more potently than cocaine, but they demonstrate behavioral profiles in animal models of psychostimulant abuse that are unlike that of cocaine. The objective of this study was to characterize the in vitro permeability, brain distribution, and pharmacokinetics of the benztropine (BZT) analogs. Transport studies of cocaine and the BZT analogs (10-4 M) were conducted across bovine brain microvessel endothelial cells. Male Sprague-Dawley rats (approximately 300 g) were administered BZT analogs (10 mg/kg) or cocaine (5 mg/kg) via the tail vein. Blood and brain samples were collected over 36 h and assayed using UV-high-performance liquid chromatography. Transport of both AHN 1-055 (2.15 x 10-4 cm/s) and JHW 007 (2.83 x 10-4 cm/s) was higher (p < 0.05) than that of cocaine (1.63 x 10-4 cm/s). The volume of distribution (12.3-30.5 l/kg) of the analogs was significantly higher than cocaine (0.9 l/kg). The BZT analogs displayed a > or =8-fold higher elimination half-life (4.12-16.49 h) compared with cocaine (0.49 h). The brain-to-plasma partition coefficients were at least two-fold higher for the BZTs versus cocaine, except for AHN 2-003. The BZT analogs are highly permeable across the blood-brain barrier and possess a pharmacokinetic profile different from that of cocaine. These characteristics, in addition to their distinctive behavioral profiles, suggest that the BZT analogs may be promising candidates for the treatment of cocaine abuse.

The reinforcing efficacy of psychostimulants in rhesus monkeys: the role of pharmacokinetics and pharmacodynamics

This study was undertaken to investigate pharmacological variables that influence the reinforcing efficacy of psychostimulants. Rhesus monkeys (n = 9) responded under a within-session, exponentially increasing, progressive ratio schedule of cocaine reinforcement. Doses of cocaine, methylphenidate (MP), cocaine analogs [(+/-)-2beta-propanoyl-3beta-(2-naphthyl)-tropane (WF-23), HD-23; (+/-)-2beta-propanoyl-3beta-(2-isopropenyl)tropane (WF-60), HD-60; and 2beta-propanoyl-3beta-(4-tolyl)-tropane (HD-11, WF-11), and 2beta-propanoyl-3beta-(4-tolyl)-tropane (HD-11, WF-11), PTT], and MP analogs [(alphaR,2R)-alpha-(2-naphthalenyl)-2-piperidineacetic acid methyl ester, HDMP-28; and (alphaR,2S)-alpha-(2-naphthalenyl)-2-pyrrolideneacetic acid methyl ester, HDMP-29] that varied in their pharmacokinetic and pharmacodynamic properties were substituted for cocaine. These drugs were chosen according to their selectivity for dopamine transporters (DAT) and 5-hydroxytryptamine (serotonin) transporters (5-HTT) as assessed in rodents and their duration of action. In addition, data pertaining to the rate of onset at DAT were collected for the cocaine analogs using an ex vivo binding assay in rodent tissue. Finally, the pharmacodynamic profile of select drugs was confirmed in primate brain tissue. All drugs had reinforcing effects except HDMP-29. The rank ordering of the peak breaking points (BPs) was cocaine = MP = HDMP-28 >or= HD-60 >or= PTT >or= HD-23 > HDMP-29. The time to peak DAT occupancy for the cocaine analogs was greater than 30 min. The potency to maintain peak BP was significantly correlated with DAT affinity. There was not a linear relationship between monoamine transporter affinity and reinforcing efficacy, but it appeared that in nonhuman primates there is a range of DAT affinity under which maximal responding is maintained. Interestingly, the 5-HTT-selective cocaine analog HD-60 functioned robustly as a reinforcer at several doses in all monkeys tested. These data question the dogma regarding the role of pharmacokinetic factors and the relative influence of DAT and 5-HTT in stimulant reinforcement.

Suppressive effects of trihexyphenidyl on methamphetamine-induced dopamine release as measured by in vivo microdialysis

Abuse of methamphetamine (MAP) and cocaine causes severe medical and social problems throughout the world. Our previous study found that trihexyphenidyl (THP), a muscarinic cholinergic receptor antagonist, specifically suppressed the rewarding properties of MAP but not of cocaine, as measured by conditioned place preference in mice. The present study examined using in vivo microdialysis whether THP differentially affects the extracellular dopamine (DA) levels in the nucleus accumbens and striatum of mice injected with MAP and cocaine in comparison with another antimuscarinic agent, scopolamine (SCP). In addition, locomotor activity was simultaneously measured during microdialysis. In vivo microdialysis experiments revealed that during the initial hour after injection of MAP (1 mg/kg) DA levels increased up to 698% in the nucleus accumbens and 367% in the striatum as compared to the basal level. These increases were reduced to 293% in the nucleus accumbens and 207% in the striatum by treatment with 5 mg/kg THP. However, SCP (3 mg/kg) had no effect on the increases in extracellular DA levels in both regions after MAP injection. Cocaine (10 mg/kg) increased DA levels during the initial hour to 254% in the nucleus accumbens and 220% in the striatum as compared to the basal level. These increases were unaffected by treatment with either THP or SCP. On the contrary, both THP and SCP enhanced the locomotor-stimulant action of MAP and cocaine. These results, together with our previous finding, suggest that THP may specifically antagonize the rewarding properties of MAP through suppression of DA release in the mesolimbic area without retarding locomotor activity.

Reinforcing strength of a novel dopamine transporter ligand: pharmacodynamic and pharmacokinetic mechanisms

Drugs that block dopamine uptake often function as positive reinforcers but can differ along the dimension of strength or effectiveness as a positive reinforcer. The present study was designed to examine pharmacological mechanisms that might contribute to differences in reinforcing strength between the piperidine-based cocaine analog (+)-methyl 4beta-(4-chlorophenyl)-1-methylpiperidine-3-alpha-carboxylate [(+)-CPCA] and cocaine. Drugs were made available to rhesus monkeys (n = 5) for i.v. self-administration under a progressive ratio schedule. Both compounds maintained responding with sigmoidal or biphasic dose-response functions (0.1-1.0 mg/kg/injection). (+)-CPCA was one-fourth as potent as cocaine and maintained fewer injections per session, at maximum. For in vitro binding in monkey brain tissue, (+)-CPCA was about one-half as potent as cocaine at the dopamine transporter (DAT), and the two compounds had similar affinities at the norepinephrine transporter. (+)-CPCA was less than 1/10 as potent as cocaine at the serotonin transporter. In ex vivo binding in rat striatum, occupancy of the DAT increased directly with dose to a maximum of approximately 80% for both compounds, and (+)-CPCA was about one-fourth as potent as cocaine. Ex vivo DAT occupancy was significantly higher for cocaine than (+)-CPCA at 2 min after injection but similar at other times. Thus, the primary differences between these compounds were in serotonin transporter affinity and the kinetics of DAT binding. These results suggest that (+)-CPCA is a weaker positive reinforcer than cocaine because it has a slower onset of action over the first few minutes after i.v. injection.

Probes for the dopamine transporter: new leads toward a cocaine-abuse therapeutic--A focus on analogues of benztropine and rimcazole

In an attempt to discover a cocaine-abuse pharmacotherapeutic, extensive investigation has been directed toward elucidating the molecular mechanisms underlying the reinforcing effects of this psychostimulant drug. The results of these studies have been consistent with the inhibition of dopamine uptake, at the dopamine transporter (DAT), which results in a rapid and excessive accumulation of extracellular dopamine in the synapse as being the mechanism primarily responsible for the locomotor stimulant actions of cocaine. Nevertheless, investigation of the serotonin (SERT) and norepinephrine (NET) transporters, as well as other receptor systems, with which cocaine either directly or indirectly interacts, has suggested that the DAT is not solely responsible for the reinforcing effects of cocaine. In an attempt to further elucidate the roles of these systems in the reinforcing effects of cocaine, selective molecular probes, in the form of drug molecules, have been designed, synthesized, and characterized. Many of these compounds bind potently and selectively to the DAT, block dopamine reuptake, and are behaviorally cocaine-like in animal models of psychostimulant abuse. However, there have been exceptions noted in several classes of dopamine uptake inhibitors that demonstrate behavioral profiles that are distinctive from cocaine. Structure-activity relationships between chemically diverse dopamine uptake inhibitors have suggested that different binding interactions, at the molecular level on the DAT, as well as divergent actions at the other monoamine transporters may be related to the differing pharmacological actions of these compounds, in vivo. These studies suggest that novel dopamine uptake inhibitors, which are structurally and pharmacologically distinct from cocaine, may be developed as potential cocaine-abuse therapeutics.

Enantioselective synthesis of S-(+)-2beta-carboalkoxy-3alpha-[bis(4-fluorophenyl)methoxy]tropanes as novel probes for the dopamine transporter

Synthesis of a series of pure S-(+)-2beta-carboalkoxy-3alpha-[bis(4-fluorophenyl)methoxy]tropanes (>99% ee) was achieved by employing a chiral amine-induced asymmetric reaction of tropinone with methyl cyanoformate as the key step. In this series, all of the S-(+)-enantiomers were 2-fold more potent than their racemic mixtures and all displayed high-affinity binding for DAT (K(i)=13-40 nM). These data support previous findings of significant divergence in structural requirements for high-affinity DAT binding among tropane-based inhibitors. Furthermore, the 2-substituent in the 3alpha-[bis(4-fluorophenyl)methoxy]tropane series is well tolerated at the DAT but not at SERT (K(i)=690-2040 nM), or muscarinic M(1) receptors (K(i)=133-4380 nM) resulting in highly selective DAT ligands that may provide new leads toward a cocaine-abuse therapeutic.

The uptake inhibitors cocaine and benztropine differentially alter the conformation of the human dopamine transporter

The binding affinity of the cocaine analog [(3)H]2 beta-carbomethoxy-3beta-(4-fluorophenyl) tropane (WIN) for the dopamine transporter (DAT) is increased by the reaction of Cys-90, at the extracellular end of the first transmembrane segment, with methanethiosulfonate (MTS) reagents. Cocaine enhances the reaction of Cys-90 with the sulfhydryl reagents, thereby augmenting the increase in binding. In contrast, cocaine decreases the reaction of Cys-135 and Cys-342, endogenous cysteines in cytoplasmic loops, with MTS reagents. Because this reaction inhibits [(3)H]WIN binding, cocaine protects against the loss of binding caused by reaction of these cysteines. In the present work, we compare the abilities of DAT inhibitors and substrates to affect the reaction of Cys-90, Cys-135, and Cys-342 with MTS ethyltrimethylammonium (MTSET). The results indicate that the different abilities of compounds to protect against the MTSET-induced inhibition of binding are attributable to differences in their abilities to attenuate the inhibitory effects of modification of Cys-135 and Cys-342 as well as to enhance the reaction with Cys-90 and the resulting potentiation of binding. The inhibitor benztropine was unique in its inability to protect Cys-135. Moreover, whereas cocaine, WIN, mazindol, and dopamine enhanced the reaction of Cys-90 with MTSET, benztropine had no effect on this reaction. These two features combine to give benztropine its weak potency in protecting ligand binding to wild-type DAT from MTSET. These results indicate that different inhibitors of DAT, such as cocaine and benztropine, produce different conformational changes in the transporter. There are differences in the psychomotor stimulant-like effects of these compounds, and it is possible that the different behavioral effects of these DAT inhibitors stem from their different molecular actions on DAT.

Cloning of dopamine, norepinephrine and serotonin transporters from monkey brain: relevance to cocaine sensitivity

We used RT-PCR to clone monoamine transporters from Macaca mulatta, Macaca fasicularis and Saimiri sciureus (dopamine transporter; DAT) and Macaca mulatta (norepinephrine transporter; NET and serotonin transporter; SERT). Monkey DAT, NET and SERT proteins were >98% homologous to human and, when expressed in HEK-293 cells, displayed drug affinities and uptake kinetics that were highly correlated with monkey brain or human monoamine transporters. In contrast to reports of other species, we discovered double (leucine for phenylalanine 143 and arginine for glutamine 509; Variant I) and single (proline for leucine 355; Variant II) amino acid variants of DAT. Variant I displayed dopamine transport kinetics and binding affinities for various DAT blockers (including cocaine) versus [3H] CFT (WIN 35, 428) that were identical to wild-type DAT (n=7 drugs; r(2)=0.991). However, we detected a six-fold difference in the affinity of cocaine versus [3H] cocaine between Variant I (IC(50): 488+/-102 nM, SEM, n=3) and wild-type DAT (IC(50): 79+/-8.2 nM, n=3, P<0.05). Variant II was localized intracellularly in HEK-293 cells, as detected by confocal microscopy, and had very low levels of binding and dopamine transport. Also discovered was a novel exon 5 splice variant of NET that displayed very low levels of transport and did not bind cocaine. With NetPhos analysis, we detected a number of highly conserved putative phosphorylation sites on extracellular as well as intracellular loops of the DAT, NET, and SERT, which may be functional for internalized transporters. The homology and functional similarity of human and monkey monoamine transporters further support the value of primates in investigating the role of monoamine transporters in substance abuse mechanisms, neuropsychiatric disorders and development of diagnostic and therapeutic agents.